Highly lamellar forms of graphite have found wide ranging industrial applicability because of their low thermal and electrical resistivity and their ability to enhance thermal and electrical conductivity when added to a low or non-conductive particulate (material). Specifically, when highly lamellar graphite is mixed with or dispersed in particulate which are non-conductive or partially electrically conductive, the thin platelets of graphite become interlaced between the base particles, thus providing a more conductive path and more uniform contact with the particles than could achieved using the same concentration of non-lamellar graphite.
Exfoliated or expanded lamellar graphite has similar enhanced characteristics and utility. Thermally exfoliated graphite ("TEG") has an accordion-like configuration of separated, stacked lamellae. Like naturally occurring lamellar graphite, delaminated, exfoliated, expanded graphite "worms" are also used for applications such as enhancing thermal or electrical conductivity in various matrices. For example, in the manufacture of alkaline dry cell batteries, delaminated exfoliated flake graphite is used in the positive electrode active material. See, e.g., U.S. Pat. No. 5,482,798 to Mototani et al., which is incorporated herein by reference. If the flake graphite can be expanded in a manner to maximize its surface area for a given mass and be successfully delaminated, greater conductivity can be attained for the positive electrode. This results in an improved discharge performance and longer useful life for the battery. Simultaneously, the amount of graphite needed to produce the electrode can be decreased, resulting in an increase of the active electrode material, MnO.sub.2.
Typically, lamellar graphite has been expanded by the intercalation of a compound into or between the interlayers of the crystal structure of the graphite. The graphite intercalation compound ("GIC") is then expanded to dramatically enlarge the spaces between the graphite interlayers. The intercalation of lamellar graphite has been studied in detail and described in numerous technical papers and patents. For example, the Mototani et al. patent identified above describes making an expanded graphite product of artificial graphite by introducing sulfuric acid into the sulfuric graphite interlayers and rapidly heating the graphite at temperatures of between 800.degree. C. and 1,000.degree. C. Similarly, U.S. Pat. No. 4,350,576 to Watanabe et al., which is incorporated by reference herein, describes an intercalation process using an electrolytic intercalation solution in which the graphite is subjected to electrolysis, dried and then heated to 1000.degree. C. to obtain an expanded graphite.
Thus, while it has been known how to expand graphite, as more uses for the material have been discovered, it has become desirable to produce such expanded graphite in commercial quantities in a more efficient and economic manner.
Accordingly, it is the object of the present invention to provide an efficient and economic method for producing expanded graphite.
This object, as well as others which will become apparent upon reference to the following description and accompanying drawing, it met by method for making expanded graphite from lamellar flake graphite comprising first providing lamellar flake graphite particles having at least a minimal purity, then intercalating the lamellar flake graphite particles with an expandable graphite intercalation compound, followed by expanding the graphite intercalation compound to exfoliate the flake graphite particles, and finally air milling the exfoliated flake graphite particles to further delaminate them.